Preparation of aluminum hydride polymorphs, particularly stabilized α-alh3

ABSTRACT

The present invention features methods for preparing stabilized α-AlH 3  and α′-AlH 3 , compositions containing these alane polymorphs, e.g., energetic compositions such as rocket propellants, and methods for using the novel polymorphs as chemical reducing agents, polymerization catalysts, and as a hydrogen source in fuel cells and batteries. The method produces stabilized alane by treating α-AlH 3  with an acidic solution that optionally contains a stabilizing agent such as an electron donor, an electron acceptor, or a compound which coordinates the Al 3+  ion.

REFERENCE TO GOVERNMENT SUPPORT

This invention was funded in part by the United States Office of NavalResearch under Contract No. N68936-98-C-0009. The United StatesGovernment has certain rights in this invention.

TECHNICAL FIELD

The present invention relates generally to aluminum hydride, or “alane,”and more particularly relates to a novel method for preparing aluminumhydride polymorphs such as α-AlH₃ and α′-AlH₃. The inventionadditionally relates to a stabilized form of α-AlH₃, to energeticcompositions, particularly propellant compositions, containing, as afuel, stabilized α-AlH₃ and/or α′-AlH₃ prepared using the method of theinvention, and to methods of using the alane polymorphs prepared hereinas chemical reducing agents, as polymerization catalysts, and as asource of hydrogen gas such as in batteries and fuel cells.

BACKGROUND

Aluminum hydride, also referred to as “alane,” is usually prepared as asolution by the reaction of lithium aluminum hydride with aluminumtrichloride. A. E. Finholt et al. (1947) J. Chem. Soc. 69:1199. Thealane-containing solution, however, is not stable, as an alane-ethercomplex precipitates from solution shortly after preparation. Inaddition, attempts to isolate the nonsolvated form of alane from theether solution result in the decomposition of the complex to aluminumand hydrogen. M. J. Rice Jr. et al. (1956) Contract ONR-494(04) ASTIANo. 106967, U.S. Office of Naval Research.

In a method for preparing non-solvated alane, alane-etherate may bedesolvated in the presence of a small amount of lithium aluminumhydride. See, for example, A. N. Tskhai et al. (1992) Rus. J. Inorg.Chem. 37:877, and U.S. Pat. No. 3,801,657 to Scruggs. Non-solvated alaneexhibits six crystalline phases, with each having different physicalproperties. The phase designated as α′-alane is essentially non-solvatedand appears under a polarizing microscope as small multiple needlesgrowing from single points to form fuzzy balls. The γ phase appears asbundles of fused needles. The γ phase is produced in conjunction withthe β phase, and both γ- and β-alane are metastable nonsolvated phasesthat convert to the more stable α-alane upon heating. The α-alane is themost stable, and is characterized by hexagonal or cubic shaped crystalsthat are typically 50-100 μm in size. The other two forms, designated δ-and ε-alane, are apparently formed when a trace of water is presentduring crystallization, and the ζ-alane is prepared by crystallizingfrom di-n-propyl ether. The α′, δ, ε and ζ polymorphs do not convert tothe α-alane and are less thermally stable than the α-form. For adiscussion of the various polymorphs, reference may be had to F. M.Brower at al. (1976) J Am. Chem. Soc. 98:2450.

Alane consists of about 10% hydrogen by weight, thereby providing ahigher density of hydrogen than liquid hydrogen. Because of the highhydrogen density and the highly exothermic combustion of aluminum andhydrogen, alane can be used as a fuel for solid propellants or as anexplosive.

Solvated alane can be synthesized by the reaction of LiAlH₄ withaluminum chloride, resulting in the alane.etherate complex (equation 1).

In an alternative synthesis, LiAlH₄ is reacted with sulfuric acid togive the alane.etherate complex (equation 2).

The AlH₃-ether complex is then treated with a mixture of LiAlH₄ andLiBH₄, and heated (equation 3).

The combination of LiBH₄/LiAlH₄ enables use of a lower processingtemperature, and α-alane is the final product after heating at 65° C.under vacuum. In an alternative synthesis, Bulychev reports that α-alanecan be prepared at pressures greater than 2.6 GPa and at temperatures inthe range of 220-250° C. B. M. Bulychev et al. (1998) Russ. J. Inorg.Chem. 43:829. Under those conditions, apparently only the α-alane formis observed.

In addition, alane can be directly synthesized by metathesis of aluminumalkyls followed by removal of the alkylaluminum byproduct in vacuum(equation 4).

Still another method of preparing nonsolvated alane is by bombarding anultrapure aluminum target with hydrogen ions. However, alane thusproduced has poor crystallinity.

One of the obstacles to large scale production of α-alane is thehandling of the diethyl ether solution of the alane•ether complex. Atconcentrations of about 0.5 M or higher and temperatures above 0° C. thealane•ether phase prematurely precipitates out of solution. In addition,α-alane can be contaminated with other phases of alane, and is notstable over time as the complex decomposes to hydrogen and aluminum.

Thus, although alane is potentially promising as a high energy densityfuel, because of its high hydrogen density and the highly exothermiccombustion of aluminum and hydrogen, the lack of a suitable method forsynthesizing alane in a stabilized form has severely limited itsapplicability.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the invention to address theabove-mentioned need in the art and provide a method for synthesizingα-alane in a stabilized form.

It is another object of the invention to provide stabilized α-alane as anovel composition of matter, prepared using the aforementioned method.

It is an additional object of the invention to provide a method forsynthesizing α′-alane.

It is a further object of the invention to provide energeticcompositions containing stabilized α-alane or α′-alane, prepared usingthe methods described herein.

It is still a further object of the invention to provide such energeticcompositions in the form of a propellant composition.

It is also an object of the invention to provide methods for usingstabilized α-alane or α′-alane, prepared using the methods describedherein, as an energy dense fuel, as a chemical reducing agent, as apolymerization catalyst, and as a source of hydrogen gas such as inbatteries and fuel cells.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing, or may be learned by practice of the invention.

In one embodiment, then, the invention relates to a method for preparingstabilized α-AlH₃ wherein: (a) an alkali metal hydride is initiallyreacted with AlCl₃ in diethyl ether to form an initial AlH₃ product andan alkali metal chloride; (b) the reaction mixture is filtered to removethe alkali metal chloride; (c) an excess of toluene is added to thefiltrate of step (b), resulting in a diethyl ether-toluene solution; (d)the diethyl ether-toluene solution is heated and distilled to reduce theamount of diethyl ether in solution, until a precipitate is formed; (e)the precipitate is isolated; and (f) the isolated precipitate is addedto an acidic solution effective to dissolve and thus remove materialsother than α-AlH₃. In a preferred embodiment, the acidic solutioncontains an α-AlH₃ stabilizing agent, e.g., a compound that coordinatesto the Al³⁺ ion, an electron donor, or an electron acceptor.

In another embodiment, the invention provides a method for synthesizingα′-AlH₃ wherein (a) an alkali metal hydride is initially reacted withAlCl₃ in diethyl ether to form an initial AlH₃ product and an alkalimetal chloride; (b) the reaction mixture is filtered to remove thealkali metal chloride; (c) an additional alkali metal hydride and anexcess of toluene are added to the filtrate of step (b), providing adiethyl ether solution containing α′-AlH₃ and optionally other AlH₃polymorphs; and (d) removing the α′-AlH₃ is from the solution.

In a further embodiment of the invention, a propellant composition isprovided containing fuel, a binder material, and an oxidizer, whereinthe fuel is a stabilized α-AlH₃ product or an α′-AlH₃ product preparedusing the aforementioned techniques. The alane polymorphs of theinvention are compatible with a wide range of binder materials,oxidizers, secondary fuels, and other propellant components, and providefor a propellant that is chemically and physically stable over anextended period of time.

The invention also provides methods for using the stabilized α-AlH₃product and the α′-AlH₃ product in other contexts. For example, an alanepolymorph as synthesized herein may be used as a chemical reducingagent, in any context wherein a hydride donor is appropriate to bringabout reduction, e.g., in reducing unsaturated carbon-carbon bonds suchas present in alkenes and alkynes, in reducing carbonyl-containingmoieties such as ketones, aldehydes, carboxylic acids, and acidchlorides, in converting halides to hydrido moieties, and the like. Thepresent alane polymorphs may also be used as polymerization catalysts,typically in catalyzing addition polymerization reactions, e.g., thepolymerization of olefins, and as a hydrogen source in fuel cells andbatteries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the relative stability of α-alaneprepared using a prior art method and α-alane prepared using the methodof the invention.

DETAILED DESCRIPTION OF THE INVENTION DEFINITIONS AND NOMENCLATURE

Before the present methods, compounds and compositions are disclosed anddescribed, it is to be understood that unless otherwise indicated thisinvention is not limited to the use of specific reagents, reactionconditions, composition components, or the like, as such may vary. It isalso to be understood that the terminology used herein is for thepurpose of describing particular embodiments only and is not intended tobe limiting.

It must be noted that, as used in the specification and the appendedclaims, the singular forms “a,” “an” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a stabilizing agent” includes mixtures of stabilizingagents, reference to “alane” can refer to more than one polymorph ofAlH₃, and the like.

The term “alane” refers to aluminum hydride, having the formula AlH₃,and includes all the polymorphs such as α-AlH₃, α′-AlH₃, δ-AlH₃, and thelike.

The term “stabilizing agent” refers to compound that tends to inhibitthe decomposition of α-alane. The stabilizing agent can be an electronacceptor, an electron donor, or a compound which coordinates to the Al³⁺ion.

The term “stabilized” when used to refer to the α-alane product of theinvention indicates that the product is substantially more stable thanα-alane products of the prior art (i.e., α-alane prepared without theuse of an acid wash workup and/or without stabilizing agents asdisclosed herein). That is, “unstabilized” α-alane rapidly decomposes tohydrogen and aluminum, while the stabilized α-alane of the inventiondoes not. “Stability” refers to both thermal stability and stability atambient temperature. With respect to thermal stability, as illustratedin FIG. 1, the “stabilized” α-alane of the invention is less than 1%decomposed after twelve days at 60° C. while decomposition of theunstabilized product at that point is virtually complete (as may beinferred from the graph).

METHOD FOR PREPARING STABILIZED α-ALH₃

In a first embodiment of the invention, a method is provided forpreparing stabilized α-AlH₃. An alane•ether complex, AlH₃•Et₂O, isinitially prepared. Aluminum chloride is dissolved in diethyl ether, andan alkali metal hydride is then added. The alkali metal hydride can be,for example, lithium hydride (LiH), sodium hydride (NaH), potassiumhydride (KH), calcium hydride (CaH₂), magnesium hydride (MgH₂), sodiumborohydride (NaBH₄), lithium borohydride (LiBH₄), lithium aluminumhydride (LiAlH₄), sodium aluminum hydride (NaAlH₄), or combinationsthereof In general, LiAlH₄, LiBH₄, and combinations thereof are thepreferred metal hydrides. The relative proportion of the alkali metalhydride to aluminum chloride is not critical; however, to assuresubstantially complete conversion of AlCl₃ to AlH₃, an excess of thealkali metal hydride is preferably used, such as a molar ratio of about3:1 to 4.5:1, preferably 3.2:1 to 4:1 (LiAlH₄:AlH₃). The reaction ispreferably carried out at a temperature of less than about 0° C. and atemperature of about −10° C. has been found to be optimal. The solution,upon substantial completion of reaction, is filtered to remove thealkali metal chloride, a by-product of the reaction. At this point, itis desirable although not essential to introduce an additional alkalimetal hydride into the reaction mixture, e.g., NaAlH₄ or NaBH₄, tofurther assist in crystallization of the product, thereby enablingremoval of impurities, particularly the slightly soluble alkali metalchloride that results along with the AlH₃ etherate complex. Theadditional alkali metal hydride may or may not be the same as the alkalimetal hydride used initially. In this case, a further filtration step isconducted. Then, a large excess of an aromatic organic solvent, e.g.,benzene, toluene, xylene, anisole, or the like, is added to thefiltrate, assisting in the removal of impurities from the reactionmixture; benzene and toluene are preferred solvents. Alternatively, thereaction can be carried out in a solvent mixture of the ether and thearomatic solvent.

Crystals of α-AlH₃ can then be obtained by first distilling theether-aromatic solvent solution to reduce the amount of ether to lessthan about 10% by volume, preferably by heating, under vacuum, and thenheating the solution until a precipitate is formed. Optimally, twocycles of heating are applied to the solution, the first at atemperature in the range of about 82-85° C. for about 1-2 hours, and thesecond at a temperature in the range of about 90-93° C. for about 20-40minutes. Crystals of α-AlH₃ prepared by this method typically reach asize of between 50-100 μm.

The α-alane precipitate is stabilized using an acid wash workup. α-Alaneis slowly added to a well-stirred dilute acid solution. The acid ischosen from hydrochloric acid, hydrogen fluoride acid, hydrogen bromideacid, phosphoric acid, perchloric acid, sulfuric acid, boric acid, andthe like, but most preferably is hydrochloric acid. The concentration ofthe acid is about 1-25% v/v, preferably about 5-15% v/v, most preferablyabout 10% v/v. The α-alane-containing dilute acid solution is filteredand rinsed, e.g., with water, an organic alcohol, and an organic,solvent. The organic alcohol may be, for example, methanol, ethanol,n-propanol, isopropanol, n-butanol, s-butanol, or t-butanol, and theorganic solvent is an ether, preferably diethyl ether. The acid washworkup removes residual impurities such as LiCl, NaBH₄, LiBH₄, LiAlH₄,and the like, as well as any remaining reactive alane polymorphs. Inaddition, the acid wash provides an Al(OH)₃ and Al₂O₃ coating on theα-alane material, thereby stabilizing the product.

In addition, the surface of the α-alane precipitate can be coated with asurface stabilizing agent. The stabilizing agent may be, for example, acompound that coordinates the Al³⁺ ion. In theory, the Al³⁺ ionscatalyze the surface decomposition of the α-alane, leading todegradation of the bulk sample. Thus, compounds that coordinate Al³⁺ions prevent the decomposition of α-alane, thereby leading to a morestable product. Suitable stabilizing agents that coordinate the Al³⁺ ionare typically polyhydric monomers and polymers and include, but are notlimited to, aluminon™ (aurintricarboxylic acid triammonium salt),8-hydroxyquinoline, and catechol. The amount of the surface stabilizerprovided on the aluminum hydride is about 0.1 to 10% of the totalweight, preferably about 1 to 5% of the total weight. The surfacestabilizers can be applied to the surface of alane during the synthesisof alane, during the acid workup process, or after the α-alane has beenisolated. Thus, for example, a surface stabilizer can be added duringthe heating cycles, during the acid wash step, or by slow evaporation ofa slurry of alane, e.g., a methanol slurry of alane, containing 1 to 5%of the stabilizers.

Additionally α-alane can be doped with other stabilizers that areelectron donors or electron acceptors. In theory, photochemical orthermal decomposition of alane is caused by the initial formation ofpositive ion/electron holes that result in hydrogen evolution. Mobileelectrons are theorized to catalyze the decomposition of alane toaluminum and hydrogen via this mechanism. Thus, the addition of electronacceptors and donors, optionally with a complexing agent that is a Lewisacid or a Lewis base, inhibits the decomposition of α-alane. Theelectron donor or acceptor stabilizers include tetrachlorobenzoquinone,diphenylamine, tetracyano-ethylene, 7,7,8,8-tetracyanoquinodimethane,tetrathiofulvalene, tetrakis(dimethylamino)-ethylene, and the like. Thetotal amount of the electron donor or acceptor stabilizers provided onthe α-alane is about 0.1 to 10% of the total weight, preferably about1-5% of the total weight. The alane is preferably doped with thestabilizers during the crystallization of the α-alane phase, although,as described above, the stabilizers can be applied to the surface ofalane during the synthesis of alane, during the acid workup process, orafter the α-alane has been isolated. In addition, band gap attenuators,that include Ti-, Si-, and P-containing derivatives, can be added duringsynthesis or to the surface. The band gap attenuators inhibit radicalreactions.

METHOD FOR PREPARING α′-ALH₃

Crystals of α′-AlH₃ can be prepared by reacting an alkali metal hydridewith AlCl₃ in a solution of diethyl ether and removing the alkali metalchloride from the reaction mixture by filtration, as described in thepreceding section. The process initially yields solvated alane. Thesolvated alane, in an ethereal solution containing an excess of analkali metal hydride or borohydride, preferably NaBH₄, NaAlH₄, or LiBH₄,most preferably LiBH₄, is heated to about 80° C. under pressure to giveprimarily the nonsolvated alane phase designated α′-AlH₃. Other AlH₃polymorphs may be present in the solution. In addition, mixtures of thenonsolvated α- and γ-AlH₃ can be prepared by slowly distilling off theether during the heating process such that the concentration of alaneapproaches the saturation level.

ENERGETIC COMPOSITIONS

A primary use of stabilized α-alane and α′-alane as prepared herein isin the manufacture of explosive and propellant compositions,particularly in the manufacture of rocket propellant compositions,including solid and solution propellants, typically solid propellants.Alane is known to be useful as an energy dense fuel in propellantformulations; however, problems with stability have arisen, as notedpreviously herein. The use of alane prepared using the method of theinvention significantly increases the stability of the propellantcomposition, and thus provides an important advance in the field.

The propellant compositions herein, in addition to alane, contain abinder material and an oxidizer. Examples of binder materials for use inpropellant applications include but are not limited to polyoxetanes,polyglycidyl azide, hydroxyl-terminated polybutadiene,polybutadieneacrylonitrileacrylic acid terpolymer, polyethers,polyglycidyl nitrate, and polycaprolactone; see, e.g., U.S. Pat. No.5,292,387 to Highsmith et al. Examples of oxidizers that may beincorporated into the compositions include, but are not limited to,ammonium nitrate (AN), phase-stabilized ammonium nitrate (PSAN),ammonium dinitramide (ADN), potassium nitrate (KN), potassiumdinitramide (KDN), sodium peroxide (Na₂O₂), ammonium perchlorate (AP),KDN-AN, a cocrystallized form of potassium dinitramide and ammoniumnitrate, cyclo-1,3,5-trimethylene-2,4,6-tri-nitramine (RDX orcyclonite), high melting explosives (HMx), diaminodinitro ethylene(DADNE), 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane(CL-20, also known as HNIW), and combinations thereof The propellant mayalso contain an additional fuel material, typically a metallic fuelcomprised of, for example, aluminum, beryllium, boron, magnesium,zirconium, or mixtures or alloys thereof. Other components forincorporation into propellant compositions include plasticizers, burnrate modifiers, ballistic additives, and the like. In general,propellant compositions of the invention are prepared by blending thestabilized alane with the oxidizer, the binder, and a curing agenteffective to cure the admixture, e.g., hexane 1,6-diisocyanate, toluenediisocyanate, isophorone diioscyanate, or the like. Proportions of fueland oxidizer can be varied to optimize performance, as will beappreciated by those skilled in the art.

OTHER USES

The present compounds may also be used as reducing agents, aspolymerization catalysts, and as a hydrogen source in fuel cells andbatteries.

Use as reducing agent: An alane polymorph as synthesized herein may beused as a chemical reducing agent in any context wherein a hydride donoris appropriate to bring about reduction, e.g., in reducing unsaturatedcarbon-carbon bonds such as present in alkenes and alkynes, in reducingcarbonyl-containing moieties such as ketones, aldehydes, carboxylicacids, esters, amides acid chlorides, in converting halides to hydridomoieties, and the like. Typically, a compound to be reduced is dissolvedin an organic solvent and reacted with the stabilized α-alane of theinvention, or with α′-alane as prepared herein, and the reaction productthen isolated and purified.

Use in Polymerization: The alane polymorphs prepared using the methodsdescribed herein may also be used as polymerization catalysts, typicallyin catalyzing addition polymerization reactions, e.g., thepolymerization of olefins. Generally, polymerization using the novelalane polymorphs as catalysts involves conventional processes whereinselected monomers are contacted with the alane polymorph under reactionconditions effective to provide the desired polymer composition.Polymerization may be carried out in solution, in a slurry, or in thegas phase. The monomer or comonomers used are preferably although notnecessarily addition polymerizable monomers containing one or moredegrees of unsaturation. Such monomers include olefinic and vinylmonomers such as ethylene, propylene, butadiene, styrene, and the like.The polymeric product resulting from the aforementioned reaction may berecovered by filtration or other suitable techniques. If desired,additives and adjuvants may be incorporated into the polymer compositionprior to, during, or following polymerization; such compounds include,for example, additional catalysts (which may or may not bepolymerization catalysts), pigments, antioxidants, lubricants andplasticizers.

Use as a hydrogen source in batteries and fuel cells: Additionally, thealane polymorphs prepared herein can be used as a hydrogen source inbatteries and fuel cells. Alane provides a higher density of hydrogenthan liquid hydrogen. Upon thermal or photochemical initiation, alane istheorized to initially produce an alane cation radical and a freeelectron. Both the cation radical and the electron can separately reactwith another alane molecule to initiate decomposition that results inthe formation of hydrogen gas and aluminum metal. Thus, light, heat, ormobile electrons can be used as initiators to catalyze thedecomposition. Accordingly, a composition containing the stabilizedα-alane and/or the α′-alane of the invention can be used for controlledrelease of hydrogen gas in a battery or fuel cell. In general, the alaneproducts of the invention will find utility in hydrogen storageelectrodes, particularly negative electrodes, in alkaline storagebatteries that comprise a positive electrode, a negative electrode, andan aqueous alkaline electrolyte. In fuel cells, electrochemical devicesfor continuous delivery of electricity wherein the devices contain afuel, e.g., a hydrogen source, and an oxidant, the alane products of theinvention will find utility as the hydrogen source.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description as well as the examples which follow are intendedto illustrate and not limit the scope of the invention. Other aspects,advantages and modifications within the scope of the invention will beapparent to those skilled in the art to which the invention pertains.

All patents, patent applications, and publications mentioned herein arehereby incorporated by reference in their entireties.

EXPERIMENTAL

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how tocarry out the methods of the invention arid prepare and use the claimedcompounds and compositions. Efforts have been made to ensure accuracywith respect to numbers (e.g., amounts, temperature, etc.) but someerrors and deviations should be accounted for. Unless indicatedotherwise, parts are parts by weight, temperature is in ° C. andpressure is at or near atmospheric.

EXAMPLE 1 SYNTHESIS OF α-ALANE

A reaction flask containing 600 mL of ether and 35 mL of a 1 M ethersolution of AlCl₃ was purged with argon. To the solution was then added6.7 g of solid LiAlH₄ over a period of 15 min. The reaction was stirredfor 15 min. and then 45 mL of a 1 M LiBH₄ in ether solution was added.The reaction was stirred for a further 30 min. at room temperature, andthen filtered to remove lithium chloride.

The filtrate was transferred to a flask containing a diethyl ethersolution of about 50 mole % of sodium borohydride. To the solution wasthen added 2.5 L of toluene and the solution was stirred for a further20 min., filtered into a reaction flask, and the reaction flask wasconnected to a distillation column. The diethyl ether was removed undervacuum by heating the reaction flask to about 60° C. until only about 7%ether was left in the reaction flask. The reaction flask was then heatedat 82-85° C. for about 2 h., and then at 90-93° C. for about 30 min.During heating, white crystalline particles formed that grow in sizeduring the heating period. Following the heating cycles, the whitecrystalline product was collected by filtration, washed twice withdiethyl ether, and dried under reduced pressure to give the α-alaneproduct.

EXAMPLE 2 SYNTHESIS OF α′-ALANE

The solvated alane was produced as in Example 1. After filtration of theether solution to remove lithium chloride, LiAlH₄ was added to thesolution, and the ethereal solution was heated to about 80° C. for 2 h.White crystalline particles form that grow in size during the heatingperiod, and were collected by filtration. The crystalline product waswashed twice with diethyl ether, and was dried under reduced pressure togive the α′-alane product.

EXAMPLE 3 STABILIZATION OF CRYSTALLINE ALUMINUM HYDRIDE

(a) The α-alane product of Example 1 was slowly added to 10% v/v ofaqueous HCl while stirring. The resulting suspension was stirred at roomtemperature for 2 hours. The solid was collected by filtration, rinsedonce with water, ethanol, and diethyl ether, and then dried to aconstant weight under reduced pressure.

(b) The procedure of part (a) was repeated with an alane stabilizationagent, aluminon, in the aqueous HCl (10% v/v).

EXAMPLE 4 STABILIZATION OF CRYSTALLINE ALUMINUM HYDRIDE WITH SURFACESTABILIZERS

The α-alane product of Example 1 was slowly added to methanol containingaluminon (4% w/v). The methanol slurry was stirred at room temperaturefor 30 min. and then methanol was slowly evaporated with gentle heatingover a period of 5 h. The solid was rinsed once with water, ethanol, anddiethyl ether, and then dried to a constant weight under reducedpressure.

EXAMPLE 5 EVALUATION OF STABILITY

The α-alane product prepared in Example 3(b) was compared with anα-alane: product prepared without an acid wash workup or surfacestabilizers, as follows. Both samples were stored at about 60° C. andthe percentage decomposition over time was evaluated using a standardTaliani test apparatus. The test results are shown in FIG. 1. As may beseen, the alane product prepared using the method of the invention wassignificantly more stable than the alane product prepared using themethod of the prior art.

The thermal stability of the α-alane product prepared in Example 3(b)was also studied by DSC and TGA. In the DSC experiments, the sample didnot exhibit the onset of an exothermic reaction until about 162° C. thatwas followed immediately with a large endothermic reaction. In the TGAexperiments, samples of α-alane of Example 3 rapidly lost weight between162-215° C. The observed mass loss was about 9 to 10% of the total mass,and corresponds to the theoretical amount of 10% hydrogen in AlH₃. In aseparate experiment, the alane did not lose mass at 70° C. in a humidenvironment after 2 days.

EXAMPLE 6 MANUFACTURE OF ENERGETIC COMPOSITIONS

The stabilized α-alane of Example 3(b), ammonium perchlorate, ammoniumdinitramide and hydroxyl-terminated polybutadiene, are blended tosubstantial homogeneity and cured with any one of a variety of curingagents including, but not limited to, hexane 1,6-diisocyanate, toluenediisocyanate and isophorone diisocyanate. The proportions of fuel andoxidizer can be varied arbitrarily within the constraint of the loadingtolerance of the binder system, so as to optimize any aspect ofperformance.

What is claimed is:
 1. A method for preparing α-AlH₃, comprising thesteps of: (a) reacting an alkali metal hydride with AlCl₃ in a solutionof diethyl ether to form an initial AlH₃ product in a reaction mixture,along with an alkali metal chloride; (b) removing the alkali metalchloride from the reaction mixture by filtration; (c) adding an excessof toluene to the filtrate resulting from step (b), providing a diethylether-toluene solution; (d) heating and distilling the diethylether-toluene solution to reduce the amount of diethyl ether in thesolution, until a precipitate is formed; (e) isolating the precipitate;and (f) adding the precipitate to an acidic solution effective todissolve and thus remove materials other than α-AlH₃.
 2. The method ofclaim 1, further comprising: (g) isolating α-AlH₃ from the acidicsolution.
 3. The method of claim 1, wherein the acidic solution of step(f) contains an α-AlH₃ stabilizing agent.
 4. The method of claim 3,wherein the α-AlH₃ stabilizing agent comprise a compound thatcoordinates to the Al³⁺ ion.
 5. The method of claim 4, wherein theα-AlH₃ stabilizing agent is aluminon.
 6. The method of claim 4, whereinthe α-AlH₃ stabilizing agent is 8-hydroxyquinoline.
 7. The method ofclaim 4, wherein the α-AlH₃ stabilizing agent is catechol.
 8. The methodof claim 3, wherein the α-AlH₃ stabilizing agent is an electron donor oran electron acceptor.
 9. The method of claim 8, wherein the electrondonor or electron acceptor is selected from the group consisting oftetrachlorobenzoquinone, diphenylamine, tetracyanoethylene,7,7,8,8-tetracyanoquinodimethane, tetrathiafulvalene andtetrakis(dimethylamino)ethylene.
 10. The method of claim 1, furtherincluding, after step (b) and prior to step (d), adding an additionalalkali metal hydride to the reaction mixture and/or the diethylether-toluene solution, followed by a further filtration step, whereinthe additional alkali metal hydride may or may not be the same as thealkali metal hydride of step (a).
 11. The method of claim 1, wherein thealkali metal hydride is LiAlH₄.
 12. The method of claim 1, wherein thealkali metal hydride is LiBH₄.
 13. The method of claim 1, wherein themolar ratio of the alkali metal hydride to AlCl₃ in step (a) is in therange of approximately 3:1 to 4.5:1.
 14. The method of claim 1, whereinthe acidic solution is an HCl solution.
 15. The method of claim 14,wherein the acidic solution is approximately 5% to 15% (w/w) HCl.